QR code: Make sure correct layering is used to make sure hint remains visible.

WireGuard/WireGuard/UI/iOS/QRScanViewController.swift

@@ -44,7 +44,7 @@ class QRScanViewController: UIViewController {
         previewLayer = AVCaptureVideoPreviewLayer(session: captureSession)
         previewLayer.frame = view.layer.bounds
         previewLayer.videoGravity = .resizeAspectFill
-        view.layer.addSublayer(previewLayer)
+        view.layer.insertSublayer(previewLayer, at: 0)
     }
 
     override func viewWillAppear(_ animated: Bool) {